It almost sounds like your coach experienced a negative pressure which sucked the duct together. One thing comes to mind... leak test. RV dealers seal a RV outside/inside as best they can. Then they put a high powered fan on a roof vent and positively pressurize the coach. They then apply soapy water on areas prone to leaking. Leaks will bubble and then they seal the leaks. However, "IF" a RV tech put the fan on up-side down it would create a negative pressure and perhaps your ducks were sucked in at this time. Perhaps a previous owner used a powerful fan similarly. I know my explanation sounds farfetched, but I have actually seen similar scenarios in industry. I watched a semi truck driver suck the sides of a tanker in when he forgot to vent while filling a large fuel tank. I also saw what happened when an electric motor turned the wrong direction (incorrectly 3 phased motor) and a large duct collapsed. Even though the actual pressure or suction is low on a gauge or monometer per square inch, when one multiples the square inch surface area the forces are tremendous once they are localized. For example, a 3 psi (positive) pressure on a semi trailer tanker equates to 50,000+ pounds of breakaway thrust at the man hole cover. A couple of inches of water column pressure (negative) on your coach could suck in the ducts if the ducks where the weakest links for air to enter in a negative pressure scenario. It’s one idea based on experience, maybe other posters will think of something else. I don't know how to fix it but... here's goofy idea, could you pressurize it to make it pop out ? My logic is, if it collapsed in the manner I suggested what would happened if the process were reversed ? I don't know if a leak test done for an extended period of time would fix it or not. Call a RV dealer and ask how much to leak test your coach, then tell them to leave the blower on extra long... Maybe others will come up with a better idea. Let us know anymore info or the end results - good luck.